Field of the Invention
The present invention relates to wireless communication and, more particularly, to a method for transmitting, by a terminal, a synchronization signal for a device-to-device (D2D) operation in a wireless communication system, and a terminal using the method.
Related Art
In an International Telecommunication Union Radio communication sector (ITU-R), a standardization of International Mobile Telecommunication (IMT)-Advanced being a next mobile communication system after a third generation has been performed. The IMT-Advanced is aimed at supporting an Internet Protocol (IP) based multi-media service with a data transmission rate of 1 Gbps in a stop and low speed moving state and a data transmission rate of 1 Gbps in a high speed moving state.
A 3rd Generation Partnership Project (3GPP) is preparing LTE-Advanced (LTE-A) being an improved one of Long Term Evolution (LTE) based on an OFDMA (Orthogonal Frequency Division Multiple Access)/SC-FDMA (Single Carrier-Frequency Division Multiple Access) transmission scheme as a system standard satisfying requirements of IMT-Advanced. The LTE-A is one important candidate for IMT-Advanced.
In recent years, there is growing interest in a Device-to-Device (D2D) technology performing direct communication between devices. In particular, the D2D is attracting attention as a communication technology for a public safety network. A commercial communication network has been rapidly changed to the LTE but a current public safety network is based on a 2G technology in a collision problem and a cost side with an existing communication standard. Request for the technology clearance and an improved service induces an effort to improve the public safety network.
The public safety network has high service requirements (reliability and security) as compared with a commercial communication network. In particular, when coverage of cellular communication is insufficient or is not used, there is a need for direct signal transmission/reception between devices, that is, a D2D operation.
The D2D operation may be signal transmission/reception between adjacent devices to have various advantages. For example, a D2D terminal may perform data communication with a high transmission rate and low delay. Further, the D2D operation may distribute traffic converged in a base station. If the D2D terminal serves as a relay, the D2D terminal may serve to extend coverage of a base station.
Meanwhile, even in a D2D operation, transmission of a synchronization signal may be required. The synchronization signal may provide a reference point in transmission, reception, decoding, etc. of a signal according to the D2D operation.
When the terminal enters a RRC connected state from a radio resource control (RRC) idle state, in the conventional technology, all D2D related signal transmission in the RRC idle state are stopped and the D2D related signals are transmitted under control of the serving cell. According to this conventional technology, when a specific terminal transmits a synchronization signal (D2DSS) for D2D operation in the RRC idle state and enters into the RRC connected state, the specific terminal may not transmit the D2DSS until receiving a setting explicitly permitting transmission of D2DSS from the serving cell. Therefore, the transmission of the D2DSS is stopped, and as a result, other terminals performing the D2D operation by performing synchronization based on the D2DSS transmitted by the specific terminal may cause a problem of synchronization for the D2D operation.